1. Field of the Invention
The present invention relates to an illumination apparatus and an illumination method in which illuminance with high uniformity is obtained at a high speed. In particular, the present invention relates to an illumination apparatus and an illumination method in which illuminance with high uniformity can be realized at a high speed, in an irradiation surface, such as a spectroscope having as large an area as a solar cell module or a solar simulator, which is applied to large area irradiation.
2. Description of the Related Art
In order to accurately measure a solar cell based on a solar simulator, illuminance in an irradiation surface needs to have high uniformity, for example, uniformity within a range of ±2% in a class A corresponding to a highest level in a standard measuring method.
FIG. 7 illustrates the outline of an optical system of a solar simulator used in standard solar cell measurement according to the related art.
As illustrated in FIG. 7, in the optical system, light emitted from a discharge lamp 101 is reflected by an elliptical reflection mirror 102 and radiated to an opening of the elliptical reflection mirror 102. The radiated light is irradiated onto an irradiation surface 107 (surface of a solar cell module) through a plane reflection mirror 103, an integrator lens 104 to make illuminance uniform, a plane reflection mirror 105, and a collimation lens 106 to collimate diffusion light.
However, in the optical system, the collimation lens 106 is used. The collimation lens 106 has spherical aberration. For this reason, illuminance ununiformity is generated in a concentric circle shape in the irradiation surface 107, and it is very difficult to improve uniformity of illuminance in the irradiation surface 107 having a large area as the solar cell module. The illuminance ununiformity can be slightly alleviated by changing a shape of the collimation lens 106, but in principle it is impossible to completely remove the illuminance ununiformity. In addition to the spherical aberration, due to multiple reflections generated between both surfaces of the collimation lens 106 and the irradiation surface 107 (surface of the solar cell module), illuminance of a central portion of an optical axis of the collimation lens 106 may become higher than those of the other portions. As a result, disturbance in the uniformity of the illuminance cannot be solved, even though all methods in the related art are used.
That is, in a solar simulator using the related art, illuminance ununiformity is large due to the spherical aberration and the multiple reflections and has a value significantly larger than values in a range of ±2% in the class A corresponding to the highest level in the standard measuring method. In particular, in regards to an influence by the multiple reflections, since the irradiation surface (surface of the solar cell) has reflectance different according to a kind thereof, even though uniformity of illuminance is maintained within a predetermined range using an arbitrary optical system with respect to a solar cell of an arbitrary kind, uniformity of illuminance may be out of the predetermined range with respect to a solar cell of a different kind. That is, a problem due to a variation factor such as different reflectance cannot be solved by a fixed device. That is, since an optical system needs to be changed according to each kind having different surface reflectance, an illumination device and an illumination method where illuminance with high uniformity is practically obtained is rarely realized, as long as the conventional method is used.
In the related art, in order to adjust illuminance of the light dispersed from the spectroscope having as large an area as the solar cell module to illuminance with high uniformity in the irradiation surface, two adjusting mechanisms that include adjustment in arrangement of an optical system including a diffraction grating for sweeping the spectral wavelength and adjustment based on an optical integrator at the back of an exit slit are adopted. However, in the first adjusting mechanism, regardless of the type of arrangement of the optical system adopted, when the diffraction grating is rotated for the purpose of sweeping the spectrum wavelength, beam uniformity obtained by the exit slit changes with respect to the wavelength and the width of the exit slit is not constant with respect to the wavelength. Therefore, even though the beam is used as incident light and uniformity of illuminance is adjusted by the optical system, it is difficult to improve uniformity of the illuminance in the irradiation surface having a large area. Even in the second adjusting mechanism, due to the color aberration or the spherical aberration of a fly eye lens used as the optical integrator, uniformity of illuminance to be obtained changes with respect to the wavelength. That is, even though the first and second adjusting mechanisms are used, a problem due to the variation factor such as the wavelength cannot be solved by the fixed device.
In the conventional spectroscope, a technology for actually measuring a change in irradiation uniformity of dispersed light due to the wavelength and adjusting the change in the uniformity does not exist. This is because the conventional spectral sensitivity measurement object is limited to a sample having an area smaller than that of the solar cell having about a square of 5 cm and uniformity of illuminance does not cause a severe problem. In recent years, a range of spectral sensitivity measurement objects is widened from a sample having as small an area as the solar cell to a sample having as large an area as the solar cell module, and an irradiation area needs to increase to about a square of several tens of centimeters. However, a technology for coping with illuminance uniformity of the spectroscope having as large an area as the solar cell module does not exist.
Reference may be made to, for example, Japanese Patent Application Laid-Open Nos. 5-183851, 8-146911, and 2002-189178.